Production of ethers from methanol

ABSTRACT

Processes and apparatus for converting methanol or the like to intermediate olefins and etherification products, such as methyl t-butyl ether by extracting crude methanol feedstock with an olefinic liquid hydrocarbon stream containing C 4  + iso-olefins. The extract phase is reacted under etherification conditions. The aqueous methanol raffinate stream is converted catalytically to olefins for recovery of C 4  + olefinic liquid hydrocarbons useful as extraction solvent.

REFERENCE TO COPENDING APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationsSer. Nos. 043,716; 043,718; and 043,729, filed Apr. 29, 1987,incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to techniques for converting alcohol feedstocks,such as crude methanol or the like, to lower ether products, eg-methyltertiary-alkyl ethers. This invention also provides a technique forconverting methanol to lower olefins. In particular, it provides acontinuous process for producing an intermediate olefinic product richin C₂ -C₅ alkenes. In view of the availability and low cost of syntheticmethanol (MeOH), primary emphasis is placed on this feedstock materialin following description of the methanol-to-olefin (MTO) process.

In its broader aspects, this invention relates to an integrated systemfor converting crude methanol to valuable products by etherifying lowerbranched olefins, such as C₄ -C₅ isoolefins. It is known thatisobutylene may be reacted with methanol over an acidic catalyst toprovide methyl tertiary butyl ether (MTBE) and isoamylenes may bereacted with methanol over an acidic catalyst to produce tertiary-amylmethyl ether (TAME). The etherification catalyst employed is preferablyan ion exchange resin in the hydrogen form. Substantially any acidiccatalyst may be employed with varying degrees of success. That is,acidic solid catalysts may be used; such as, sulfonic resins, phosphoricacid modified kieselguhr, silica alumina and acid zeolites. Those ethershaving the formula CH₃ -O-R, where R is an isoalkyl radical, areparticularly useful as octane improvers for liquid fuels, especiallygasoline.

Increasing demand for high octane gasolines blended with lower aliphaticalkyl ethers as octane boosters and supplementary fuels has created asignificant demand for tert-alkyl ethers, especially the C₅ to C₇ methylalkyl ethers, such as methyl tertiary butyl ether (MTBE) and tertiaryamyl methyl ether (TAME). Methanol may be readily obtained from coal bygasification to synthesis gas and conversion of the synthesis gas tomethanol by well-established industrial processes. As an alternative,the methanol may be obtained from natural gas by other conventionalprocesses, such as steam reforming or partial oxidation to make theintermediate syngas. Crude methanol from such processes usually containsa significant amount of water, usually in the range of 4 to 20 wt%;however, the present invention is useful for removing water in lesseramounts or greater.

Proceses for converting lower oxygenates such as methanol tohydrocarbons are known (eg-methanol-to -olefins -MTO), and have becomeof great interest in recent times because they offer an attractive wayof producing liquid hydrocarbon fuels, especially gasoline, from sourceswhich are not of liquid petroleum origin. In particular, they provide away by which methanol can be converted to a major amount of C₂ -C₅olefins and a minor amount of gasoline boiling range products in goodyields.

It is main object of the present invention to provide a novel andeconomic technique for removing excess water from crude methanolfeedstocks, including novel operating methods and equipment for treatingthese oxygenate feedstocks prior to conversion to olefins andetherification.

SUMMARY OF THE INVENTION A continuous technique has been found forconverting crude methanol to methyl t-alkyl ethers in a catalyticreaction zone with acid etherification catalyst comprising methods andmeans for:

(a) contacting a crude methanol feedstock containing a minor amount ofwater with a liquid hydrocarbon extraction stream rich in C₄ +iso-olefinic hydrocarbons under extraction conditions favorable toselective extraction of the methanol, thereby providing an extractliquid stream rich in methanol and an aqueous raffinate streamcontaining unextracted methanol;

(b) charging the extract liquid stream with C₄ + olefinic hydrocarbonand extracted methanol substantially free of water to a catalyticetherification reaction zone under process conditions for convertingmethanol and iso-olefin to predominantly methyl tertiary-alkyl ether;

(c) catalytically converting the aqueous raffinate stream in contactwith methanol-to-olefin catalyst to produce a mixture of C2-C5 orheavier olefins, including C₄ + iso-olefinic component;

(d) contacting at least a portion of etherification reaction effluentfrom step (b) with water to recover methanol from the effluent; and

(e) recovering a product stream rich in methyl tertiary-alkyl ether.

These and other objects and features of the invention will be understoodfrom the following description and in the drawing.

DRAWING

The single figure of the drawing is a schematic olefins production andetherification process flowsheet depicting the present invention.

DETAILED DESCRIPTION

The crude methanol commercially available from syngas processes maycontain, for instance 4 to 17 wt% water, which must be removed,preferrably to a methanol purity of about 99.8 wt%. It has been foundthat dry methanol from crude feedstock methanol can be recovered byliquid extraction with light olefinic liquid extractant, such as butenesand C₅ + light olefinic naphtha. The typical feed ratio range is about 5to 20 parts hydrocarbon extractant per part by volume of recoveredmethanol.

Referring to the drawing, a continuous stream of crude methanol (MeOH)feedstock is introduced via conduit 10 with a stream of hydrocarbonliquid extractant and optional recycle introduced via conduit 12 to aninlet of extraction separation unit 14. These streams are contactedunder liquid extraction conditions to provide an aqueous raffinatephase. An aqueous stream containing a major amount of the water presentin the crude feedstock is withdrawn via conduit 16. The lighter organicextract phase containing hydrocarbon extraction solvent and extractedmethanol is recovered from extraction unit 14 via conduit 18, combinedwith additional C₄ + olefins from line 19 and introduced undertemperature and process conditions suitable for conversion of methanolin contact with etherification catalyst in etherification reactor system30, including a conventional reactor unit and effluent distillationunit. From reactor system 30 a first effluent stream containingunreacted methanol and C₄ - light hydrocarbons leaves via line 32 and ispassed to effluent washer vessel 40, where it is contacted with washwater introduced via line 42 for extraction of unreacted methanol fromfirst light hydrocarbon product stream 44, rich in unreacted C₄hydrocarbons. The aqueous raffinate stream 16 consists essentially ofwater, partitioned methanol and a minor amount of hydrocarbon. Theaqueous raffinate containing unextracted oxygenate is passed to a MTOreaction zone 50 under process conditions to convert oxygenate topredominantly olefinic hydrocarbons. This is followed by separating MTOreaction effluent to recover aqueous liquid byproduct, gas rich in C₃ ⁻hydrocarbons, and an intermediate liquid product stream comprising C₄ +hydrocarbons. The raffinate methanol is converted to a mixture ofolefins in MTO system 50, including a catalytic reactor andfractionation unit to provide light hydrocarbon product streams 52, 54;byproduct water effluent stream 56 and liquid intermediate olefinsstream 58, rich in C₄ + olefin isomers. Optionally, light olefinic MTOeffluent containing ethene, propene, etc., may be further upgraded inreactor system 60 to supplement the liquid C4+ stream 58 with additionalC4-C7 olefins for instance from conduit 62. The combined liquid stream58 is apportioned according to process demand between etherificationfeedstream 19 and extraction solvent stream 59, which may be combinedwith a portion of recycled wet methanol from wash unit 40 via conduit46.

EXTRACTION UNIT OPERATION

The typical preferred crude feedstock material is methanol containingabout 4 to 17% by weight water. The extraction contact unit may be astirred multi-stage vertical extraction column adapted for continuousoperation at elevated pressure. Any suitable extraction equipment may beemployed, including cocurrent, cross-current or single stage contactors,wherein the liquid methanol feedstock is intimately contacted with asubstantially immiscible liquid hydrocarbon solvent, which may be amixture of C₄ ⁺ aliphatic components including lower alkanes, n-alkenesor relatively pure isoalkenes, such as isobutylene, etc. This unitoperation is described in Kirk-Othmer Encyclopedia of ChemicalTechnology (Third Ed.), 1980, pp. 672-721. Other equipment forextraction is disclosed in U.S. Pat. Nos. 4,349,415 (DeFilipi et al) and4,626,415 (Tabak). The methanol extraction step can be performedadvantageously in a countercurrent multistage design, such as a simplepacked column, rotating disk column, agitated column with baffles ormesh, or a series of single stage mixers and settlers.

Etherification Operation

The reaction of methanol with isobutylene and isoamylenes at moderateconditions with a resin catalyst is known technology, as provided by R.W. Reynolds, et al., The Oil and Gas Journal, June 16, 1975, and S.Pecci and T. Floris, Hydrocarbon Processing, December 1977. An articleentitled "MTBE and TAME--A Good Octane Boosting Combo", by J. D. Chase,et al., The Oil and Gas Journal, Apr. 9, 1979, pages 149-152, discussesthe technology. A preferred catalyst is a polyfunctional ion exchangeresin which etherifies and isomerizes the reactant streams. A typicalacid catalyst is Amberlyst 15 sulfonic acid resin. MTBE and TAME areknown to be high octane ethers. The article by J. D. Chase, et al., Oiland Gas Journal, Apr. 9, 1979, discusses the advantages one can achieveby using these materials to enhance gasoline octane. The octane blendingnumber of MTBE when 10% is added to a base fuel (R+O=91) is about 120.For a fuel with a low motor rating (M+O=83) octane, the blending valueof MTBE at the 10% level is about 103. On the other hand, for an (R+O)of 95 octane fuel, the blending value of 10% MTBE is about 114.

MTO REACTOR SYSTEM

Recent developments in zeolite technology have provided a group ofmedium pore siliceous materials having similar pore geometry. Mostprominent among these intermediate pore size zeolites is ZSM-5, which isusually synthesized with Bronsted acid active sites by incorporating atetrahedrally coordinated metal, such as Al, Ga, or Fe, within thezeolytic framework. These medium pore zeolites are favored for acidcatalysis; however, the advantages of ZSM-5 structures may be utilizedby employing highly siliceous materials or crystalline metallosilicatehaving one or more tetrahedral species having varying degrees ofacidity. ZSM-5 crystalline structure is readily recognized by its X-raydiffraction pattern, which is described in U.S. Pat. No. 3,702,866(Argauer, et al.), incorporated by reference.

Catalyst versatility permits the same zeolite to be used in bothmethanol dehydration and olefin formation. While it is within theinventive concept to employ substantially different catalysts in pluralstages, it is advantageous to employ a standard ZSM-5 having a silicaalumina molar ratio of 70:1 or greater in a once-through fluidized bedunit to convert feedstock oxygenate to hydrocarbons.

The MTO catalysts preferred for use herein include the crystallinealuminosilicate zeolites having a silica to alumina ratio of at least12, a constraint index of about 1 to 12. Representative of the ZSM-5type zeolites are ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 andZSM-48. ZSM-5 is disclosed and claimed in U.S. Pat. No. 3,702,886 andU.S. Pat. No. Re 29,948; ZSM-11 is disclosed and claimed in U.S. Pat.No. 3,709,979. Also, see U.S. Pat. No. 3,832,449 for ZSM-12; U.S. Pat.No. 4,076,979. Also, See U.S. Pat. No. 3,832,449 for ZSM-12; U.S. Pat.No. 4,076,842 for ZSM-23; U.S. Pat. No. 4,016,245 for ZSM-35; and U.S.Pat. No. 4,375,573 for ZSM-48. The disclosures of all patents citedherein are incorporated herein by reference. The medium pore shapeselective catalysts are sometimes known as porotectosilicates or"pentasil" catalysts, and may have a pore size of about 5 to 7Angstroms. In addition to the preferred aluminosilicates,silicoaluminophosphate, gallosilicate, borosilicate, ferrosilicate and"silicalite" materials may be employed.

Various zeolitic catalysts are useful for converting methanol and otherlower aliphatic alcohols or corresponding ethers to olefins. Recentinterest has been directed to a catalytic process for convertingmethanol over ZSM-5 and related catalysts to valuable hydrocarbons richin ethene and C₃ ⁺ alkenes. Various processes are described in U.S. Pat.Nos. 3,894,107 (Butter et al); 3,928,483; 4,025,575; 4,252,479 (Chang etal); 4,025,572 (Lago); 4,328,384 (Daviduk et al); and 4,547,616 (Avidanet al); incorporated herein by reference. It is generally known that MTOprocesses can be optimized to produce a major fraction of C₂ -C₄olefins; however, a significant C₅ ⁺ byproduct may be coproduced. Priorprocess techniques for increasing lower olefin selectivity have providedfor controlled deposition of coke byproduct on the catalyst surface.

Methanol may be first subjected to a dehydrating step, using a catalystsuch as gamma-alumina, to form an equilibrium mixture of methanol,dimethyl ether (DME) and water. This mixture is then passed at elevatedtemperature and pressure over a catalyst such as ZSM-5 zeolite forconversion to the hydrocarbon products. Water may be removed from themethanol dehydration products prior to further conversion tohydrocarbons and the methanol can be recycled to the dehydration step,as described in U.S. Pat. No. 4,035,430. Removal of the water isdesirable because the catalyst may tend to become deactivated by thepresence of excess water vapor at the reaction temperatures employed;but this step is not essential.

ZSM-5 type pentasil zeolites are particularly useful in the MTO processbecause of their regenerability, long life and stability under theextreme conditions of MTO operations. Usually the zeolite crystals havea crystal size from about 0.02 to 2 microns or more, with large crystalson the order of 0.1-1 micron being preferred. In order to obtain thedesired particle size for fluidization in the turbulent regime, thezeolite catalyst crystals are bound with a suitable inorganic oxide,such as silica, alumina, etc. to provide a zeolite concentration ofabout 5 to 90 wt. %. It is advantageous to employ a particle size rangeconsisting essentially of 1 to 150 microns. Average particle size isusually about 20 to 100 microns, preferably 40 to 80 microns. Thecatalyst in the fluidized bed reactor is maintained at an average acidcracking activity (alpha value) of about 1 to 15, preferably about 3 to8, on a coke-free basis. The average coke content is less than 15 weightpercent, preferably about 5-10 wt. % of the clean-burned catalyst. Bycontrolling the catalytic properties of the system, the selectivity toproduce C₂ -C₅ olefins can be enhanced.

In a preferred embodiment, aqueous methanol raffinate 16 is passed tothe MTO reactor system 50 with wet methanol wash stream 48 (optionalrecycle). The combined feedstock and recycle is conducted at atemperature of about 275°-525° C., preferrably about 475°-500° C., and apressure of about 100-1000 kPa to the MTO catalytic reactor. Effluentfrom the reaction zone is passed to a separation zone for recovery oflight gas streams 52, 54, byproduct water and a liquid hydrocarbonstream 58 containing a mixture of butenes, isobutylene, pentenes,isoamylene, and C6+ gasoline range aromatics and aliphatics. It may bedesirable to upgrade C2-C3 olefins (see U.S. Pat. No. 4,579,999, Gouldet al) in an optional intermediate stage reactor system 60, withrecovery of C3- light offgas and an upgraded olefinic hydrocarbondstream 62 containing etherifiable isoolefins, such as isobutylene andisoamylene.

The present invention is particularly advantageous in the economicdewatering of crude methanol, thus avoiding expensive andenergy-intensive prefractionation by distillation. Various modificationscan be made to the system, especially in the choice of equipment andnon-critical processing steps. While the invention has been described byspecific examples, there is no intent to limit the inventive concept asset forth in the following claims.

We claim:
 1. A continuous process for converting crude methanol tomethyl tert-alkyl ethers in a catalytic reaction zone with acidetherification catalyst comprising the steps of:(a) contacting a crudemethanol feedstock containing a minor amount of water with a liquidhydrocarbon extraction stream rich in C₄ + iso-olefinic hydrocarbonsunder extraction conditions favorable to selective extraction of themethanol, thereby providing an extract liquid stream rich in methanoland an aqueous raffinate stream containing unextracted methanol; (b)charging the extract liquid stream with C₄ + olefinic hydrocarbon andextracted methanol substantially free of water to a catalyticetherification reaction zone under process conditions for convertingmethanol and iso-olefin to predominantly methyl tertiary-alkyl ether;(c) catalytically converting the aqueous raffinate stream in contactwith methanol-to-olefin catalyst to produce a mixture of C2-C5 orheavier olefins, including C₄ + iso-olefinic component; (d) contactingat least a portion of etherification reaction effluent from step (b)with water to recover methanol from the effluent; and (e) recovering aproduct stream rich in methyl tertiary-alkyl ether.
 2. The process ofclaim 1 wherein the etherification catalyst comprises acidic ionexchange resin.
 3. The process of claim 1 wherein the feedstock consistsessentially of methanol and about 4 to 20 wt% water, and wherein theextraction liquid comprises C₄ -C₅ iso-olefin.
 4. The process of claim 1wherein the methanol-to-olefin catalyst consists essentially of HZSM-5and wherein the crude feedstock contains about 4 to 20 wt. % water, andabout 10 to 60 wt. % dimethyl ether.